Abstract: The packaging of fresh beef in a modified atmosphere is a complex and dynamic system in which various factors interact for the maintenance of the quality of the product. The concentration of gases surrounding the product and the storage conditions could decisively influence the stability of the color and microbiological quality of the meat conditioned under modified atmosphere. Therefore, the optimization of a packaging system to maintain the quality of fresh meat involves or is determined by several variables: the gaseous composition, the ratio between the gas volume and the product weight, the gas permeability of the package, time and temperature of the storage, among others. Through the experiments which were carried out, it was possible to evaluate various packaging systems, institutional and conventional, and to understand how important variables of the process interact during the storage and retail exposure of the meat. The literature review presented in Chapter 1 was achieved by trying to summarize in a comprehensive and readable form until the present date, looking for the easiest way to apply the appropriate gaseous mixtures for a specific commercial purpose, and also to support the professionals in the meat industry in the selection of most appropriate packaging systems to effectively meet their interests and needs. In Chapter 2 sirloin and rib eye steaks were conditioned to the primary packaging in a modified atmosphere 30% of CO2 and variable concentrations of CO (0.2-0.4%), O2 (0- 21%) and N2(balanced) with the goal of determining the best mixture containing CO with the ability to match the optimum microbiological growth inhibition and color stability. In Chapter 3 the interrelationship between the discoloration and off-odor were evaluated by determining the concentrated residual effects of oxygen combined with high levels of CO2 (30 and 100%) which results in the reoxygenation and microbiological quality of fresh meat wrapped with or without CO (0.2%). In the final experiment, Chapter 4, the lowest concentration of CO (0.2%) with the ability to produce an attractive color, was associated with high concentrations of CO2 (60% and 99.8%) to extend the shelf life of the steaks from three types of beef muscles (longissimus dorsi, gluteus medium and psoas major) and of ground meat (triceps brachii)